Synchronous rectification circuit, corresponding device and method

ABSTRACT

A sense terminal is configured to sense a drain-to-source voltage of a field effect transistor and a drive terminal is configured to drive the gate terminal of the field effect transistor to alternatively turn the field effect transistor on and off to provide a rectified current flow in the field effect transistor channel. A comparator is configured to perform a comparison of the drain-to-source voltage of the field effect transistor with a reference threshold and to detect alternate downward and upward crossings of the reference threshold and the drain-to-source voltage. A PWM signal generator is configured to drive the gate terminal of the field effect transistor to turn the field effect transistor on and off as a result of the alternate downward and upward crossings of the reference threshold by the drain-to-source voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to Italian Patent Application No.102018000004743, filed on Apr. 20, 2018, which application is herebyincorporated herein by reference.

TECHNICAL FIELD

The description relates to a synchronous rectification circuit, as wellas a corresponding device and method.

BACKGROUND

In various power conversion systems, such as AC/DC and DC/DC converters,field effect transistors or FETs (MOSFET transistors, for instance)driven by control logic can replace rectifier diodes.

This technique, oftentimes referred to as synchronous rectification(SR), is found to improve converter efficiency. Resorting to SRfacilitates reducing conduction losses insofar as the (rectified) outputcurrent flows through the MOSFET channel instead of the rectificationdiode, with power losses correspondingly reduced.

Such a power converter continues to operate also if a synchronousrectification FET is not driven. This is because rectification is stillprovided by an internal body diode (essentially body-to-drain, withbody-to-source being irrelevant insofar as this is shorted out by aninternal body-to-source connection).

The body diode (which is intrinsic to most FET types) may howeverexhibit poor performance: properly driving a (MOS)FET, when the bodydiode is forward biased, may increase system efficiency by about 3%-4%.

SUMMARY

Despite the intensive activity in that area, further improved solutionsare desirable. Embodiments can contribute in providing such improvedsolutions.

The description relates to synchronous rectification. One or moreembodiments can be applied, for instance, to a variety of AC/DC andDC/DC converters. Converters for use in battery chargers for electronicdevices, USB power delivery (USB-PD) arrangements, adapters arenon-limiting examples of such applications.

One or more embodiments may relate to a corresponding device, e.g., aMOSFET-based synchronous rectifier in a battery charger for electronicdevices, a USB power delivery (USB-PD) arrangement an adapter and so on.

One or more embodiments may relate to a corresponding method.

One or more embodiments may be based on the recognition that, whilesuited to be implemented with analog components (for instance dedicatedICs), synchronous rectification implemented in digital form facilitatesreducing the number of components, achieving improved flexibility indevising control procedures and a higher tolerance to noise.

One or more embodiments may provide adaptive synchronous (SR)implementation.

One or more embodiments may be implemented with a microcontroller (suchas, for instance, an STM32 microcontroller as available with companiesof the ST group) by using only the internal peripherals therein.

In one or more embodiments, an (internal) comparator may trigger a timerwhich in turn generates a pulse-width modulation (PWM) signal. Thecomparator can be reconfigured on-the-fly by a direct memory access(DMA) channel to trigger PWM shut-down (otherwise a second internalcomparator can be used). An analog-to-digital converter (ADC) channelcan be used to sample the drain-to-source voltage Vds of the (MOS)FETafter PWM turn-off, then the synchronous rectification (SR) controllogic can change the comparator threshold by using, for instance, adigital-to-analog converter or DAC channel.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments will now be described, by way of example only,with reference to the annexed figures, wherein:

FIG. 1 comprises two portions, FIGS. 1A and 1B, and is representative ofthe possible replacement of diode rectification with synchronousrectification by means of field effect transistors (FETs) such aMOSFETs,

FIG. 2 is a time diagram exemplary of certain signals possibly involvedin synchronous rectification with FETs,

FIG. 3 is a block diagram exemplary of a possible context of use ofembodiments,

FIGS. 4 and 5 are further time diagrams exemplary of certain signalspossibly involved in synchronous rectification with FETs, and

FIG. 6 is a flow chart exemplary of possible operation of embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In the ensuing description, one or more specific details areillustrated, aimed at providing an in-depth understanding of examples ofembodiments of this description. The embodiments may be obtained withoutone or more of the specific details, or with other methods, components,materials, etc. In other cases, known structures, materials, oroperations are not illustrated or described in detail so that certainaspects of embodiments will not be obscured.

Reference to “an embodiment” or “one embodiment” in the framework of thepresent description is intended to indicate that a particularconfiguration, structure, or characteristic described in relation to theembodiment is comprised in at least one embodiment. Hence, phrases suchas “in an embodiment” or “in one embodiment” that may be present in oneor more points of the present description do not necessarily refer toone and the same embodiment. Moreover, particular conformations,structures, or characteristics may be combined in any adequate way inone or more embodiments.

The references used herein are provided merely for convenience and hencedo not define the extent of protection or the scope of the embodiments.

As noted, in various power conversion systems, such as AC/DC and DC/DCconverters, field effect transistors, namely FETs (MOSFET transistors,for instance) driven by control logic can replace rectifier diodes. Thistechnique, oftentimes referred to as synchronous rectification (SR) isfound to improve converter efficiency.

The FIGS. 1A and 1B are exemplary of the possibility of replacing one ormore rectification diodes (for instance two rectification diodes D1, D2in FIG. 1A) with respective corresponding FETs (MOSFET transistors, forinstance) F1 and F2 in FIG. 1B within the framework of a rectifiercircuit.

This may occur, for instance, at the secondary side of a transformer T,with the MOSFET transistors F1 and F2 alternatively turned on (that is,made conductive) and off (that is, made non-conductive) under thecontrol of a control unit CU so that a (rectified) current may flow inchannels of the MOSFET transistors F1 and F2.

The rectifier circuit architecture in FIG. 1 includes two diodes D1, D2(FIG. 1A) or two MOSFET transistors F1, F2 (FIG. 1B) coupled to thesecondary winding of a transformer T and having cascaded thereto a RClow pass filter suited to provide a (rectified) voltage signal V_(out)to a load L.

It will be appreciated that the rectifier circuit architecture in FIG. 1is just exemplary of a wide variety of rectifier circuits to which SRcan be applied. Therefore, the circuit architecture of FIG. 1 is not tobe construed, even indirectly, as confining the scope of theembodiments.

One or more embodiments may comprise (digital) controller circuitssuited to be coupled to field effect transistors (for instance, F1 andF2 in FIG. 2).

A conventional field effect transistor (FET) such as, for instance, aMOSFET comprises a channel between source and drain terminals as well asa body diode and a gate terminal configured to control electricalcurrent flow in the field effect transistor channel.

As noted, by resorting to SR, conduction losses can be reduced thanks tothe output current I_(out) flowing through a (MOS)FET channel instead ofa rectification diode, so that the power loss is decreased fromP_(loss_diode)=V_(d)·I_(out) (where V_(d) is the voltage drop across thediode) to P_(loss_MOSFET)=R_(ds_on)·I_(out) ² (where R_(ds_on) is thedrain-source resistance in the “on” or conductive state) which may be(very) low for SR MOSFETs.

As similarly noted, such a power converter continues to operate also ifa synchronous rectification FET is not driven. This is becauserectification is still provided by an internal body diode, which,however, may exhibit poor performance: properly driving such a MOSFETwhen the body diode is forward biased may increase system efficiency byabout 3%-4%.

A problem underlying synchronous rectification (SR) using FETs (MOSFETswill be primarily referred to in the following for simplicity) isdetecting the MOSFET body diode conduction in order to be able toturn-off the MOSFET channel in a fast and reliable way, thus avoidingcurrent inversion, which may cause MOSFET failure.

This may apply, for instance, to those systems where a digitalcontroller has no prior information about diode conduction (e.g.,because the power converter is driven by another IC). To increase theefficiency of the system, a control logic should desirably be able toreduce progressively the conduction times of the MOSFET body diode, forinstance via adaptive SR driving.

It is observed that digital solutions can be conceived where thedrain-source voltage V_(DS) of a SR MOSFET can be sensed to detect bodydiode conduction by using additional hardware (i.e., comparators, etc.)to measure the conduction time. Based on this measure, the duration ofthe PWM “on” time can be adjusted to avoid a fast turn-off mechanism ifthe diode conduction time changes abruptly. Such an implementation canbe resorted to, for instance, in those topologies (e.g., LLC converters)where the turn-on time may be already known to the digital controller.

One or more embodiments may rely on a MOSFET turn-on and turn-offmechanism where (only) internal resources of a digital controller may beused to detect the start and the end of conduction of the body diode ofa FET such as a MOSFET and drive the FET consequently. This mayfacilitate performing an adaptive SR algorithm which increases theefficiency of the converter in a simple and reliable way.

As exemplified in the diagram of FIG. 2, the drain-source voltage V_(DS)voltage (possibly conditioned—e.g., converted to digital—as desired, inmanner known per se) can be sensed and sent to an comparator (e.g.,internal to the controller CD) and compared to a thresholdV_(TH_ON_OFF), set by the controller.

This may occur within the framework of a circuit as exemplified in FIG.3.

There a digital controller CD is shown coupled to a power converter CPcomprising a rectifier arrangement of one or more SR FETs such as MOSFETtransistors.

As noted, one or more embodiments may be applied to a wide variety ofrectifier circuits adopting synchronous rectification.

For that reason, FIG. 3 refers for simplicity to a converter circuitblock CP, which may include one or more rectification FETs whose V_(DS)voltage can be sensed with current conduction in the FET channelcontrolled via a PWM signal applied to the FET gate. An example of sucha converter circuit block CP is shown in FIG. 1B.

For the sake of simplicity, FIG. 3 refers to a single FET whose VDSvoltage can be sensed via a conventional voltage sensor VS at a sensenode 10 of the digital controller CD and where current conduction in theFET channel can be controlled via a PWM signal applied to the FET gatevia a drive node 12 of the digital controller CD.

For the sake of explanation it will be assumed that a “high” state ofthe PWM signal is intended to cause the FET to be conductive (turned on)and that a “low” state of the PWM signal is intended to cause the FET tobe non-conductive (turned off). Of course, adopting such an option isnot mandatory.

FIG. 3 illustrates a (digital) comparator 14 in the controller CD where,as discussed in the following, the drain-source voltage VDS sensed atnode 10 is compared with a reference value (e.g., V_(TH_ON_OFF)), withthe result of comparison at 14 used to drive a PWM generator block 16having an output coupled to the drive node 12 which controls (rectified)current flow (I_(SR)) in the FET channel.

In one or more embodiments, rising and falling edges of V_(DS) at thecomparator 14 (that is crossings of the threshold(s) of the comparator14 by V_(DS)) can be used to trigger the PWM generator block 16 togenerate at node 12 the PWM signal for driving a FET in the converterCP.

For instance, as exemplified in FIG. 2, when the voltage V_(DS)decreases below the threshold of the comparator 14 (falling edge at thecomparator output—see point A in FIG. 2—which is indicative of the bodydiode conducting), the PWM generation block 16 (which may comprise atimer) is triggered to generate a PWM signal, designated PWM_SR (e.g.,“high”), after a programmed delay (turn-on delay TOD).

The signal PWM_SR is kept high for a minimum on-time (blanking window ofthe comparator BT) to avoid false triggers.

A result of VDS rising again above the threshold (rising edge at thecomparator output—see point B in FIG. 2—which means that the body diodeis going to be reverse biased), the comparator output triggers via theblock 16 the shutdown of PWM signal PWM_SR (see FIG. 2, right handside), at a comparator trigger time CT earlier with respect to a normalpulse duration NP of the PWM signal.

Intervals of body diode conduction at points A and B are indicated as DC(body diode conduction).

It will be appreciated that, in one or more embodiments, various optionsmay be resorted to in order to facilitate this kind of operation.

For instance, a single comparator with a single threshold can be usedwith the comparator configured to trigger alternatively, PWM signalturn-on and turn-off. Hysteresis (possibly programmable) may be presentand kept at a low level in order to facilitate switching at a desiredthreshold. Avoiding undesired abrupt switching may be facilitated by theBlanking window feature BT discussed previously.

In one or more embodiments, two comparators with a single threshold maybe used, so that the same threshold can be used for turn-on (triggeredby a first comparator) and turn-off (triggered by a second comparator),so that no reconfiguration is required.

In one or more embodiments, two comparators with two (different)thresholds may be used, so that a first threshold can be used forturn-on and a second (variable) threshold can be used for turn-off.

In this description of exemplary embodiments, a single (adaptive)threshold V_(TH_ON_OFF) will be considered in order to make thepresentation simpler and facilitate understanding of the embodiments.

FIG. 4—where entities like entities already discussed in connection withFIG. 2 are indicated with like references (a related description willnot be repeated here for brevity)—shows that if the PWM signal PWM_SR isturned-off too early, a (still) positive current I_(SR) through the FETwill causes a non-negligible body diode conduction (DC, right hand sideof FIG. 4). This implies low converter efficiency, so that the benefitsof SR are at least partially lost.

FIG. 5—where entities like entities already discussed in connection withFIGS. 2 and 4 are again indicated with like references (a relateddescription will not be repeated here for brevity)—shows that if,conversely, the PWM signal PWM_SR is turned-off too late, the FET willbe forced to conduct even with the diode reverse-biased (DC, right handside of FIG. 5) and an ensuing negative current I_(SR) can damage theMOSFET and cause its failure.

Moreover, after FET turn-off, a voltage ringing of V_(DS) may force thebody diode to conduct again, thus producing an undesired turn on: seeUTO in FIG. 5). The comparator 14 may be triggered again to generate anundesired PWM signal PWM_SR (FIG. 5, bottom right) losing efficiency andkeeping the FET on for a minimum “on” time.

One or more embodiments may thus adopt an adaptive SR control logicwhich facilitates reducing body diode conduction time by making thecomparator threshold VTH_ON_OFF adaptive.

One or more embodiments may thus check if the threshold (V_(TH_ON_OFF))of the comparator 14 is well tuned. This may occur by sampling thevoltage V_(DS) with a programmable sampling delay SD from PWM turn-offusing the previous threshold value, and comparing it with a fixed value.

An analog-to-digital converter (ADC) channel can be used to sample thedrain-to-source voltage Vds (for instance at times T-ADC in FIGS. 4 and5) of the (MOS)FET after PWM turn-off (time indicated CT in FIGS. 4 and5) and the synchronous rectification SR control logic can change thethreshold(s) in the comparator 14 by using, for instance, andigital-to-analog converter or DAC channel.

The programmed delay facilitates achieving a condition where the(MOS)FET is completely turned-off because it can consider both turn-offdelay and propagation delay introduced by gate drivers (both delays canbe known and do not vary over time).

The value thus obtained being found to be below the preset threshold(see right hand side of FIG. 4) means that the MOSFET body diode isstill conducting and the PWM is turned-off too early because the voltagedrop on the MOSFET is equal to the forward voltage of body diode. Inthis case the threshold of the comparator 14 can be increased to achievea later (delayed) MOSFET turn-off.

Otherwise, the acquired value being above the preset threshold (seeright hand side of FIG. 5) suggests that MOSFET may have been “forced”to conduct even with a reverse current and the PWM is turned-off toolate. This may be related to the fact that a small safe interval, inwhich the diode conducts (little notch in the V_(DS) waveform), is notpresent and V_(DS) rises (too) quickly. In this case the threshold ofthe comparator can be decreased to turn-off the MOSFET earlier.

The sampling delay from PWM turn-off (times CT in FIGS. 2, 4 and 5)gives also the time duration of this safe body diode conductioninterval.

In the block diagram of FIG. 3, reference 18 denotes a circuit blockwhich is triggered (via a line T) by a signal provided by the PWMgenerator 16 to provide a delayed acquisition of the signal V_(DS) atthe input node 10.

Reference 20 denotes an adaptive SR logic acting in cooperation with thedelayed acquisition block 18 and with a circuit block 22 which controls(adaptively) the threshold(s) of the comparator 14.

In one or more embodiments, the threshold of the comparator 14 can beincreased and decreased within an expected range [COMP_(THMIN);COMP_(THMAX)] that depends on the characteristics of the sensing circuitand can be acquired with some measurements.

A small capacitor can be added in the sensing circuit VS to reduce theslope of V_(DS) in sensing the associated waveform and obtain a betterthreshold regulation.

The flowchart of FIG. 6 is exemplary of an adaptive synchronousrectification (SR) procedure which can be performed at each MOSFETturn-off or at a lower frequency (in this latter case, the comparatorthreshold(s) is/are kept constant over more PWM cycles).

The block 100 in the flow-chart of FIG. 6 is representative of MOSFETturn-off, triggered as a result of comparing, e.g., in the comparator14, the (conditioned) sensed value for V_(DS) with a “previous”threshold.

The block 102 is exemplary of V_(DS) being sampled after a fixed delay(e.g., T-ADC) added to the FET turnoff time CT taken as starting time.This operation can occur in an automatic manner in a digital controller(for instance, a STM32 microcontroller as available with companies ofthe ST group) even without resorting to software instructions.

For instance (as a function of the type of controller) the possibilityexists of adding a certain (predetermined) delay starting from an(external) event such as the comparator trigger to an internal timer ofthe controller.

The timer may be the same as used for generating the PWM signal (PWMgenerator block 16) in advanced microcontrollers (a so-called“autodelayed mode” in e.g., STM32F334).

Alternatively, one may use a further timer which starts counting at atrigger event (e.g., the falling edge of the PWM signal.

If the microcontroller does not include internal trigger connections onemay use a further pin of the microcontroller coupled with the PWM signaland start the further timer at a falling edge (see, for instance theline T in FIG. 3).

As a result of the timer reaching a certain count value (that is, afterT-ADC has lapsed), a (sampling) ADC is triggered.

In one or more embodiments the comparator trigger can be stored in aregister in a timer (for instance, in the memory of themicrocontroller).

With the measurement result received from the ADC triggered at a desiredtime, in a block 104 a check can be as to whether V_(DS) as sampled ishigher than a preset desired value, e.g., Vds_TH.

If the check at 104 yields a positive outcome (Y), a decreased value forthe threshold (e.g., V_(TH_ON_OFF)) in the comparator 14 is calculatedat 106 and a check is performed at 108 as to whether the value thuscalculated (not yet set) is lower than a minimum respective value, e.g.,COMP_(THMIN).

If the check at 108 yields a positive outcome (Y) the minimum value isselected for the comparator threshold(s).

If the check at 104 yields a negative outcome (N), an increased valuefor the threshold (e.g., V_(TH_ON_OFF)) in the comparator 14 iscalculated at 112 and a check is performed at 114 as to whether thevalue thus calculated (not yet set) is higher than a maximum respectivevalue, e.g., COMP_(THMAX).

If the check at 114 yields a positive outcome (Y) the maximum value isselected for the comparator threshold(s).

Starting from any of:

the block 110;

a negative outcome (N) of the check at 108;

the block 116;

a negative outcome (N) of the check at 114,

a (new) value for the threshold (e.g., V_(TH_ON_OFF)) of the comparatorcan be set at 118 as dictated by these previous acts.

In one or more embodiments, a circuit may comprise a controller (e.g.,CD) configured for coupling to a field effect transistor (e.g., asincluded in CP in FIG. 3) having a field effect transistor channelbetween source and drain terminals as well as a body diode and a gateterminal configured to control electrical current flow in the fieldeffect transistor channel. The controller may comprise a sensing port(e.g., 10), a comparator (e.g., 14), and a PWM signal generator (e.g.,16).

The sense terminal (e.g., node 10) is configured to sense (e.g., VS) thedrain-to-source voltage (e.g., V_(DS)) of the field effect transistor aswell as a drive terminal (e.g., node 12) configured to drive the gateterminal of the field effect transistor to alternatively turn the fieldeffect transistor on and off to provide a rectified current flow in thefield effect transistor channel.

The comparator (e.g., 14) is coupled to the sensing port. The comparatoris configured to perform a comparison of the drain-to-source voltage ofthe field effect transistor with at least one reference threshold (e.g.,V_(TH_ON_OFF): as noted, a single threshold is discussed for simplicity)and detect alternate downward (e.g., A) and upward (e.g., B) crossingsof the at least one reference threshold by the drain-to-source voltage.

The PWM signal generator (e.g., 16) is coupled to the comparator circuitand the drive terminal. The PWM signal generator is configured to drive(e.g., PWM_SR) the gate terminal of the field effect transistor to turnthe field effect transistor on and off as a result of the alternatedownward and upward crossings of the at least one reference threshold bythe drain-to-source voltage.

In one or more embodiments, the PWM signal generator may be configuredto turn the field effect transistor on with a certain delay (e.g., TOD)to the respective crossings of the at least one reference threshold bythe drain-to-source voltage.

In one or more embodiments, the PWM signal generator may be configuredto keep the field effect transistor turned on for on-time intervals inexcess of a lower on-time threshold (e.g., BT).

One or more embodiments may comprise an acquisition circuit block (e.g.,18) and an adaptive network (e.g., 20, 22). The acquisition circuitblock (e.g., 18) coupled (e.g., at T) to the PWM signal generator in thecontroller. The acquisition circuit block is sensitive to field effecttransistor turn off times (e.g., CT), the acquisition circuit blockcoupled to the sensing port and configured to sense the drain-to-sourcevoltage of the field effect transistor with a certain delay (e.g.,T-ADC) to field effect transistor turn off times. The adaptive network(e.g., 20, 22) is coupled to the acquisition circuit block in thecontroller. The adaptive network is configured to control the fieldeffect transistor turn off times as a function of the drain-to-sourcevoltage of the field effect transistor sensed with a certain delay atthe acquisition circuit block.

One or more embodiments may comprise the adaptive network configured tocompare (e.g., 104 in FIG. 6) the drain-to-source voltage (V_(DS)) ofthe field effect transistor sensed with a certain delay (T-ADC) at theacquisition circuit block with an acquisition threshold value (forinstance, V_(DS_TH)) and cause the field effect transistor turn offtimes to occur later resp. earlier (e.g., due to a change in thethreshold in the comparator 14) as a result of the drain-to-sourcevoltage of the field effect transistor sensed with a certain delay atthe acquisition circuit block being below resp. above the acquisitionthreshold value.

In one or more embodiments the adaptive network may be arrangedintermediate the acquisition circuit block and the comparator, theadaptive network configured to vary (see, e.g., 100 to 118 in FIG. 6)the at least one reference threshold as a function of thedrain-to-source voltage of the field effect transistor sensed with acertain delay at the acquisition circuit block.

In one or more embodiments, the controller may comprise a digitalcontroller.

In one or more embodiments, a device such as, for instance, a converterfor use in battery chargers for electronic devices, USB power delivery(USB-PD) arrangements, adapters and so on, may comprise a rectificationnetwork (e.g., T, L, C, F1, F2 in FIG. 1) comprising at least one fieldeffect transistor having a field effect transistor channel betweensource and drain terminals as well as a body diode and a gate terminalconfigured to control electrical current flow in the field effecttransistor channel. The device can also include a circuit according toone or more embodiments, having the sensing port coupled to the at leastone field effect transistor and configured (e.g., VS) to sense thedrain-to-source voltage of the at least one field effect transistor andthe drive terminal coupled to the gate terminal of the at least onefield effect transistor to alternatively turn the at least one fieldeffect transistor on and off to provide a rectified current flow in thechannel thereof.

In one or more embodiments, a method of driving a field effecttransistor having a field effect transistor channel between source anddrain terminals as well as a body diode and a gate terminal configuredto control electrical current flow in the field effect transistorchannel. The method may comprise sensing the drain-to-source voltage ofthe field effect transistor and driving the gate terminal of the fieldeffect transistor to alternatively turn the field effect transistor onand off to provide a rectified current flow in the field effecttransistor channel, performing a comparison of the drain-to-sourcevoltage of the field effect transistor with at least one referencethreshold and detecting alternate downward and upward crossings of theat least one reference threshold by the drain-to-source voltage, anddriving the gate terminal of the field effect transistor by turning thefield effect transistor on and off as a result of the alternate downwardand upward crossings of the at least one reference threshold by thedrain-to-source voltage.

Without prejudice to the underlying principles, the details andembodiments may vary, even significantly, with respect to what has beendescribed by way of example only, without departing from the extent ofprotection.

The extent of protection is determined by the annexed claims.

What is claimed is:
 1. A circuit configured to be coupled to a fieldeffect transistor that has a channel between source and drain terminalsas well as a body diode and a gate terminal configured to controlelectrical current flow in the field effect transistor channel, whereinthe circuit comprises: a sense terminal configured to sense adrain-to-source voltage of the field effect transistor; a drive terminalconfigured to drive the gate terminal of the field effect transistor toalternatively turn the field effect transistor on and off to provide arectified current flow in the field effect transistor channel; acomparator coupled to the sense terminal, the comparator configured toperform a comparison of the drain-to-source voltage of the field effecttransistor with a reference threshold and to detect alternate downwardand upward crossings of the reference threshold and the drain-to-sourcevoltage; and a PWM signal generator coupled to the comparator and thedrive terminal, the PWM signal generator configured to drive the gateterminal of the field effect transistor to turn the field effecttransistor on and off as a result of the alternate downward and upwardcrossings of the reference threshold by the drain-to-source voltage. 2.The circuit of claim 1, wherein the PWM signal generator is configuredto turn the field effect transistor on with a delay relative to thedownward crossings of the reference threshold and the drain-to-sourcevoltage.
 3. The circuit of claim 1, wherein the PWM signal generator isconfigured to keep the field effect transistor turned on for on-timeintervals in excess of a lower on-time threshold.
 4. The circuit ofclaim 1, further comprising an acquisition circuit block coupled to thePWM signal generator, wherein the acquisition circuit block is sensitiveto field effect transistor turn off times, the acquisition circuit blockcoupled to the sense terminal and configured to sense thedrain-to-source voltage of the field effect transistor with a delayrelative to field effect transistor turn off times.
 5. The circuit ofclaim 4, further comprising an adaptive network coupled to theacquisition circuit block, the adaptive network configured to controlthe field effect transistor turn off times as a function of thedrain-to-source voltage of the field effect transistor sensed with thedelay at the acquisition circuit block.
 6. The circuit of claim 5,wherein the adaptive network is configured to compare thedrain-to-source voltage of the field effect transistor sensed with thedelay at the acquisition circuit block with an acquisition thresholdvalue to cause the field effect transistor turn off times to occurearlier as a result of the drain-to-source voltage of the field effecttransistor sensed with the delay at the acquisition circuit block beingabove the acquisition threshold value.
 7. The circuit of claim 5,wherein the adaptive network is configured to compare thedrain-to-source voltage of the field effect transistor sensed with thedelay at the acquisition circuit block with an acquisition thresholdvalue to cause the field effect transistor turn off times to occur lateras a result of the drain-to-source voltage of the field effecttransistor sensed with the delay at the acquisition circuit block beingbelow the acquisition threshold value.
 8. The circuit of claim 5,wherein the adaptive network is arranged between the acquisition circuitblock and the comparator, the adaptive network configured to vary thereference threshold as a function of the drain-to-source voltage of thefield effect transistor sensed with the delay at the acquisition circuitblock.
 9. The circuit of claim 1, wherein the circuit operates as adigital controller.
 10. A circuit comprising: a rectification circuitcomprising a field effect transistor that has a channel between sourceand drain terminals and a gate terminal configured to control electricalcurrent flow in the channel; a sense terminal configured to sense adrain-to-source voltage of the field effect transistor; a drive terminalcoupled to the gate terminal of the field effect transistor toalternatively turn the field effect transistor on and off to provide arectified current flow in the field effect transistor channel; acomparator circuit coupled to the sense terminal, the comparator circuitconfigured to perform a comparison of the drain-to-source voltage of thefield effect transistor with a reference threshold and to detectalternate downward and upward crossings of the reference threshold andthe drain-to-source voltage; and a PWM signal generator coupled to thegate terminal of the field effect transistor, the comparator circuit andthe drive terminal, the PWM signal generator configured to drive thegate terminal of the field effect transistor to turn the field effecttransistor on and off as a result of the alternate downward and upwardcrossings of the reference threshold by the drain-to-source voltage. 11.The circuit of claim 10, wherein the rectification circuit furthercomprises: a transformer having a first secondary winding terminal and asecond secondary winding terminal, the first secondary winding terminalcoupled to the source of the field effect transistor; an inductor havinga first terminal and a second terminal, the first terminal of theinductor coupled to the drain of the field effect transistor; and acapacitor having a first terminal and a second terminal, the firstterminal of the capacitor coupled to the second terminal of theinductor.
 12. The circuit of claim 11, further comprising a second fieldeffect transistor coupled to the transformer.
 13. The circuit of claim12, wherein the second field effect transistor has a source coupled tothe second secondary winding terminal and the second terminal of thecapacitor, the second field effect transistor also having a draincoupled to the drain of the field effect transistor and the firstterminal of the inductor.
 14. The circuit of claim 12, wherein thesecond field effect transistor has a source coupled to the secondsecondary winding terminal and a drain coupled to the drain of the fieldeffect transistor and the first terminal of the inductor, wherein thesecond terminal of the capacitor is coupled to the source of the fieldeffect transistor and to the first secondary winding terminal.
 15. Thecircuit of claim 12, wherein the comparator circuit comprises a singlecomparator and the reference threshold comprises a single threshold. 16.The circuit of claim 12, wherein the reference threshold exhibits ahysteresis characteristic based on turning on and off of the fieldeffect transistor.
 17. The circuit of claim 12, wherein the comparatorcircuit comprises two comparators coupled to the reference threshold.18. The circuit of claim 12, wherein the comparator circuit comprisesfirst and second comparators, the first comparator coupled to thereference threshold and the second comparator coupled to a secondreference threshold.
 19. The circuit of claim 10, further comprising anacquisition circuit block coupled to the PWM signal generator, whereinthe acquisition circuit block is sensitive to field effect transistorturn off times, the acquisition circuit block coupled to the senseterminal and configured to sense the drain-to-source voltage of thefield effect transistor with a delay relative to field effect transistorturn off times.
 20. The circuit of claim 19, further comprising anadaptive network coupled to the acquisition circuit block, the adaptivenetwork configured to control the field effect transistor turn off timesas a function of the drain-to-source voltage of the field effecttransistor sensed with the delay at the acquisition circuit block.
 21. Amethod of driving a field effect transistor that has a channel betweensource and drain terminals and a gate terminal that is configured tocontrol electrical current flow in the field effect transistor channel,the method comprising: sensing a drain-to-source voltage of the fieldeffect transistor; comparing a drain-to-source voltage of the fieldeffect transistor with a reference threshold; detecting alternatedownward and upward crossings of the reference threshold by thedrain-to-source voltage; and driving the gate terminal of the fieldeffect transistor by turning the field effect transistor on and off as aresult of the alternate downward and upward crossings of the referencethreshold by the drain-to-source voltage, the driving causing arectified current flow in the field effect transistor channel.